Optical ring resonators

A computer-simulated ring resonator depicting continuous wave input at resonance.

An optical ring resonator, or micro-ring resonator (MRR) is a set of waveguides in which at least one is a closed loop coupled to light input and output. The input and output can be, but are not limited to being, waveguides. Geometries behind optical ring resonators are the same as those behind whispering galleries except that they use light and obey the properties behind constructive interference and total internal reflection. When light of the resonant wavelength is passed through the closed loop from the input waveguide, the light builds up in intensity over multiple round-trips owing to constructive interference and is coupled to the output waveguide. Because only a narrow wavelength range will be at resonance within the loop geometry, the optical ring resonator functions as a filter. Additionally, two or more ring waveguides can be coupled to each other to form an add/drop optical filter or to send light in a preferred direction.